In the electronics industry, the continuing goal has been to reduce the size of electronic devices, such as camcorders and portable telephones, while increasing performance and speed. Integrated circuit devices for complex electronic systems typically are incorporated into packages that are then mounted on printed circuit wiring boards (PCBs) using solder bump technology
Solder bump technology involves printing a solder paste on input/output pads on the integrated circuit packages and heating the solder paste to a temperature equal to or greater than that of its melting point in a “reflow” process.
The reflow process has been long used to connect electronic components having input/output pins onto a PCB and many types of reflow ovens have been developed. However, PCB reflow for solder bump technology is a more complex operation that, as a result, requires an oven that is larger, more complex and more expensive than is necessary for conventional reflow.
The solder-reflow step involves four phases: preheat, activate, reflow, and cooling. First, in the preheat phase the solder paste is warmed to a temperature that is just below the melting point of the solder, below about 183° C., in a nitrogen atmosphere. In the activate phase, the flux that is used to adhere the solder to the solder pads is activated to remove oxide on the pads, and the temperature of the substrate and the solder are allowed to become more uniform and stabilized. During this activate phase the temperature of the solder and the substrate is nearly constant or may increase slightly. In the reflow phase, the temperature is caused to increase rapidly and exceed the melting point so as to melt the solder and wet the solder pads. To prevent oxidation, a huge volume of nitrogen is required to purge the oven used for the reflow. Finally, in the cooling phase, the solder and the substrate are allowed to cool to a temperature well below the melting point of the solder such that the solder solidifies and the reflow process is complete.
A problem with the solder bump process is that the solder paste contains the flux, and when the solder melts, it forms bumps that contain empty voids resulting from the dissolution of the flux. It has been found that in excess of 30% of the volume of a solder bump may be empty voids, which weakens the solder connection and increases the electrical resistance of the solder bump.
Another problem with the solder bump process is that a large amount of time is required to start and complete the nitrogen atmosphere formation and purge. The time requirement slows down the throughput of the system.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.